(每日一词）insidious

Insidious  /ɪnˈsɪd.i.əs/ adj. 暗中为害的；隐伏的

meaning:(of something unpleasant or dangerous) gradually and secretly causing harm 

前缀：in-向内

词根：sid- 坐，同sit (inside)

后缀：-ous 形容词后缀

e.g. High blood pressure is an insidious condition which has few symptoms. 高血压是一种鲜现症状的潜伏性疾病。